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Building Java Programs

Chapter 1Lecture 1-1: Introduction; Basic Java Programs

reading: 1.1 - 1.3

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Programmingprogram: A set of instructions

to be carried out by a computer.

program execution: The act ofcarrying out the instructions contained in a program.

programming language: A systematic set of rules used to describe computations in a format that is editable by humans.

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Course principlesLots of resources and people who want to help you

Deliberate topic progression

Coherence between lectures, sections, labs, homework, exams

What you do will determine what you learn

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Take this course if you…… like solving tricky problems

… like building things

… (will) work with large data sets

… are curious about how Facebook, Google, etc work

… have never written a computer program before

… are shopping around for a major142 is a good predictor of who will enjoy and succeed in CSE

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Why Java?Relatively simple

Object-oriented

Pre-written software

Platform independent (Mac, Windows…)

Widely used#1 in popularity ie

http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html

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Compiling/running a program1. Write it.

code or source code: The set of instructions in a program.

2. Compile it.• compile: Translate a program from one language to another.byte code: The Java compiler converts your code into a

format named byte code that runs on many computer types.

3. Run (execute) it.output: The messages printed to the user by a program.

source code compile

byte code run

output

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A Java programpublic class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); System.out.println(); System.out.println("This program produces"); System.out.println("four lines of output"); }}

Its output:Hello, world!

This program producesfour lines of output

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Structure of a Java programpublic class name { public static void main(String[] args) { statement;; statement;; ...... statement;; }}

Every executable Java program consists of a class,that contains a method named main,

that contains the statements (commands) to be executed.

class: a program

statement: a command to be executed

method: a named groupof statements

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System.out.printlnA statement that prints a line of output on the console.

pronounced "print-linn"sometimes called a "println statement" for short

Two ways to use System.out.println :

• System.out.println("text");

Prints the given message as output.

• System.out.println();

Prints a blank line of output.

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Names and identifiersYou must give your program a name.

public class GangstaRap {

Naming convention: capitalize each word (e.g. MyClassName)Your program's file must match exactly (GangstaRap.java)

includes capitalization (Java is "case-sensitive")

identifier: A name given to an item in your program.must start with a letter or _ or $subsequent characters can be any of those or a number

legal: _myName TheCure ANSWER_IS_42 $bling$ illegal: me+u 49ers side-swipe Ph.D's

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Keywordskeyword: An identifier that you cannot use because it

already has a reserved meaning in Java.

abstract default if private this boolean do implements protected throw break double import public throws byte else instanceof return transient case extends int short try catch final interface static void char finally long strictfp volatile class float native super while const for new switch continue goto package synchronized

i.e., You may not use char or while for the name of a class.

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Syntaxsyntax: The set of legal structures and commands that can

be used in a particular language.Every basic Java statement ends with a semicolon ;The contents of a class or method occur between { and }

syntax error (compiler error): A problem in the structure of a program that causes the compiler to fail.Examples:Missing semicolonToo many or too few { } braces Illegal identifier for class nameClass and file names do not match ...

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Syntax error example1 public class Hello {2 pooblic static void main(String[] args) {3 System.owt.println("Hello, world!")_ 4 }5 }

Compiler output: Hello.java:2: <identifier> expected pooblic static void main(String[] args) { ^ Hello.java:3: ';' expected } ^ 2 errors

The compiler shows the line number where it found the error.The error messages can be tough to understand!

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Stringsstring: A sequence of characters to be printed.

Starts and ends with a " quote " character. The quotes do not appear in the output.

Examples:

"hello""This is a string. It's very long!"

Restrictions:May not span multiple lines."This is nota legal String."

May not contain a " character."This is not a "legal" String either."

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Escape sequencesescape sequence: A special sequence of characters used

to represent certain special characters in a string.

\t tab character\n new line character\" quotation mark character\\ backslash character

Example:System.out.println("\\hello\nhow\tare \"you\"?\\\\");

Output:\hellohow are "you"?\\

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QuestionsWhat is the output of the following println statements?

System.out.println("\ta\tb\tc");System.out.println("\\\\");System.out.println("'");System.out.println("\"\"\"");System.out.println("C:\nin\the downward spiral");

Write a println statement to produce this output:

/ \ // \\ /// \\\

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AnswersOutput of each println statement:

a b c\\'"""C:in he downward spiral

println statement to produce the line of output:

System.out.println("/ \\ // \\\\ /// \\\\\\");

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QuestionsWhat println statements will generate this output?

This program prints aquote from the Gettysburg Address.

"Four score and seven years ago,our 'fore fathers' brought forth onthis continent a new nation."

What println statements will generate this output?

A "quoted" String is'much' better if you learnthe rules of "escape sequences."

Also, "" represents an empty String.Don't forget: use \" instead of " !'' is not the same as "

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Answersprintln statements to generate the output:

System.out.println("This program prints a");System.out.println("quote from the Gettysburg Address.");System.out.println();System.out.println("\"Four score and seven years ago,");System.out.println("our 'fore fathers' brought forth on");System.out.println("this continent a new nation.\"");

println statements to generate the output:

System.out.println("A \"quoted\" String is");System.out.println("'much' better if you learn");System.out.println("the rules of \"escape sequences.\"");System.out.println();System.out.println("Also, \"\" represents an empty String.");System.out.println("Don't forget: use \\\" instead of \" !");System.out.println("'' is not the same as \"");